1. Field of the Invention
The present invention relates to a substrate having pins serving as input/output terminals arranged in a standing condition, and more particularly to a substrate having enhanced joining strength between pins and a substrate.
2. Description of Related Art
There is known an upright pin-joined (pin grid array) substrate serving as input/output terminals arranged in a standing condition. An example of such a prior art upright pin-joined substrate is shown in FIG. 4, which is a partially enlarged sectional view of an upright pin-joined substrate 201. The upright pin-joined substrate 201 includes a substrate 203 having a substantially rectangular plate-like shape and a number of pins 221 arranged on the substrate 203 in a standing condition.
The substrate 203 includes a dielectric layer 205 having wiring layers (not shown) formed in the interior and on the surface thereof, and a number of pin pads 209 exposed through a solder resist layer 207 on the side of a main surface 203A.
The pin 221 includes a substantially cylindrical shaft portion 221A and an enlarged diameter portion 221B formed at the lower end of the pin 221 and swelling substantially hemispherically toward the corresponding pin pad 209. The pin 221 is fixedly joined to the substrate 203 such that the enlarged diameter portion 221B is fixedly joined to the pin pad 209 by means of solder HA.
The upright pin-joined substrate 201 has the following feature. Since the enlarged diameter portion 221B of the pin 221 assumes a substantially hemispherical shape, a larger amount of solder HA can reside between the enlarged diameter portion 221B and the pin pad 209 as compared with, for example, a pin having a flat headed, enlarged diameter portion. Since the enlarged diameter portion 221B is substantially hemispherical, stress imposed on the pin 221 does not concentrate at a specific point, but tends to be absorbed by the entire joint portion.
Accordingly, as compared with an upright pin-joined substrate using pins each having a flat headed, enlarged diameter portion, the upright pin-joined substrate 201 can provide enhanced joining strength between the pin 221 and the substrate 203 (pin pad 209).
However, when the amount of solder HA for joining the pin 221 and the pin pad 209 is small, the joining strength between the pin 221 and the pin pad 209 becomes insufficient. As a result, when stress is imposed on the pin 221, the joint portion may fracture.
Specifically, as shown in FIG. 4, when solder HA is joined through fusion to merely a spherical surface 221BK of the enlarged diameter portion 221B and is thus constricted, stress imposed on the pin 221 tends to concentrate in the vicinity of the constricted portion of solder HA, potentially causing fracture of solder HA at the constricted portion.
Also, when the amount of solder HA for joining the pin 221 and the pin pad 209 is excessively large, stress imposed on the pin 221 may cause fracture of the joint portion.
Specifically, as illustrated with the dashed line in FIG. 4, solder HA spreads, through wetting, over the entire shaft side plane 221BH of the enlarged diameter portion 221B up to the shaft portion 221A in such a manner as to form a large fillet F on the shaft side plane 221BH.
In this case, when stress is imposed on the pin 221, the pin 221 is unlikely to deform, because of a large amount of solder HA joined through fusion to the shaft portion 221A thereof; i.e., the pin 221 is unlikely to absorb the stress. As a result, the stress is imposed directly on the joint portion, potentially causing fracture of the joint portion.
The pin 221 having the substantially hemispherical, enlarged diameter portion 221B generally has a higher barycenter of the enlarged diameter portion 221B than does a pin having a flat headed, enlarged diameter portion. As a result, when the amount of solder HA is large, stress imposed on the pin 221 tends to cause fracture of solder HA at the joint portion.
Because of the spherical surface 221BK, the pin 221 is thicker in the enlarged diameter portion 221B than is a pin having a flat headed, enlarged diameter portion. Thus, when the large fillet F is formed on the shaft side plane 221BH, the strength of the joint portion becomes rather weak.
The present invention provides a substrate having at least one pin comprising a substrate, a pin, and solder. The substrate has a main surface and a substantially plate-like shape and comprises a pin pad exposed on the main surface. The pin comprises a shaft portion and an enlarged diameter portion (a flange) greater in diameter than the shaft portion and formed at one end of the shaft portion. The enlarged diameter portion comprises a shaft side plane located on the same side as the shaft portion, and a spherical surface swelling in opposition to the shaft portion. The solder is adapted to join the pin pad and at least the enlarged diameter portion of the pin. The solder extends from the pin pad beyond the circumferential edge of the shaft side plane of the enlarged diameter portion and spreads on the shaft side plane through wetting in such a manner as not to reach the shaft portion. Alternatively, the solder extends from the pin pad beyond the circumferential edge of the shaft side plane of the enlarged diameter portion and spreads, through wetting, over the entire shaft side plane up to the shaft portion in such a manner as to form a fillet on the shaft side plane with an angle formed between a fillet surface and the shaft side plane not exceeding 50 degrees.
According to the present invention, the solder for joining the pin and the pin pad extends from the pin pad beyond the circumferential edge of the shaft side plane of the enlarged diameter portion and spreads on the shaft side plane through wetting in such a manner as not to reach the shaft portion. Alternatively, the solder for joining the pin and the pin pad extends from the pin pad beyond the circumferential edge of the shaft side plane of the enlarged diameter portion and spreads, through wetting, over the entire shaft side plane up to the shaft portion in such a manner as to form a fillet on the shaft side plane. The fillet is relatively small such that the angle formed between the surface of the fillet and the shaft side plane is not greater than 50 degrees.
When the solder extends beyond the circumferential edge of the shaft side plane onto the shaft side plane through wetting, the solder extending between the pin pad and the shaft side plane of the enlarged diameter portion forms a fillet assuming substantially the form of a truncated cone. Thus, a sufficient amount of solder is present for joining the pin and the pin pad, thereby suppressing fracture of the joint portion that would otherwise result from insufficiency of solder when stress is imposed on the pin.
Even in the case where a solder fillet is formed on the shaft side plane of the enlarged diameter portion, when stress is imposed on the pin, the pin itself is deformed to some extent to thereby absorb the stress, because the fillet is relatively small. Thus, stress imposed on the joint portion is lessened. As a result, the joint portion becomes unlikely to fracture.
The pin having the hemispherical, enlarged diameter portion has a higher barycenter of the enlarged diameter portion than does a pin having a flat headed, enlarged diameter portion, because of the hemispherical profile of the enlarged diameter portion. However, since the fillet is relatively small, even when stress is imposed on the pin, the joint portion becomes unlikely to fracture.
As described above, the upright pin-joined substrate of the present invention exhibits resistance to fracture potentially caused by stress imposed on a pin, to thereby enhance reliability.
The pin is not particularly limited, so long as the pin includes the above-mentioned shaft portion and enlarged diameter portion. The shaft portion and the enlarged diameter portion may be integrally formed into a pin from pure copper (oxygen-free copper), 194-alloy (copper alloy), Koval (iron-nickel-cobalt alloy), 42-alloy (iron-nickel alloy), or a like alloy. Alternatively, the shaft portion and the enlarged diameter portion may be joined into a pin, or a portion of the enlarged diameter portion may be joined to the remaining portion of the enlarged diameter portion to thereby form a pin (for example, a substantially hemispherical member is joined to a flat headed pin). Further alternatively, a plurality of members of different materials may be joined into a pin (for example, a substantially hemispherical member made of a brazing filler metal, such as eutectic silver brazing filler metal, is joined to a flat headed pin made of Koval).
The substrate is not particularly limited, so long as a pin pad is exposed on the main surface thereof. The substrate may be a resin substrate formed of resin or a composite material which contains resin, or a ceramic substrate formed of alumina, glass ceramic, or a like ceramic material. Alternatively, the substrate may be a printed wiring substrate or a multilayer printed wiring substrate having a conductive layer(s), such as a wiring layer(s).
Solder used in the present invention can be selected as appropriate from among known types of solder in consideration of, for example, heat resistance of a substrate. Herein, the term xe2x80x9csolderxe2x80x9d refers to a soft brazing filler metal having a melting point of not higher than 450xc2x0 C.
When a gold, hereinafter xe2x80x9cAuxe2x80x9d plating layer is formed on the surface of a pin, the Au plating layer enhances wettability with respect to solder. As a result, solder tends to spread broadly on the shaft side plane of the pin through wetting or tends to creep high along the shaft portion of the pin. Thus, when a pin coated with an Au plating layer is to be used, in order to limit spreading-through-wetting of solder or creeping-up-through-wetting of solder, use of solder exhibiting relatively low wettability with respect to the Au plating layer is preferred; specifically, use of solder containing antimony, hereinafter xe2x80x9cSb,xe2x80x9d such as solder of the tin-antimony, hereinafter xe2x80x9cSnxe2x80x94Sb,xe2x80x9d -type or solder of the lead-tin-antimony, hereinafter xe2x80x9cPbxe2x80x94Snxe2x80x94Sbxe2x80x9d-type.
Solder having an Sb content of not less than 3 wt % is particularly preferred. Through employment of an Sb content of not less than 3 wt %, solder wettability diminishes clearly to thereby reliably prevent spreading-through-wetting of solder or creeping-up-through-wetting of solder. In order to avoid difficulty in soldering associated with extreme diminution in solder wettability, the Sb content is preferably not greater than 15 wt %. Examples of such solder include solder of the 95Snxe2x80x945Sb type (melting point 235xc2x0 C. to 240xc2x0 C.) and solder of the 82Pbxe2x80x9410Snxe2x80x948Sb type (melting point 255xc2x0 C. to 257xc2x0 C.). When the thickness of the Au plating layer formed on the surface of a pin is not less than 0.04 xcexcm, solder wettability is particularly enhanced. As a result, solder tends to spread broadly or creep up high, through wetting. Thus, selection of the above-mentioned solder is important in order to limit spreading-through-wetting of solder or creeping-up-through-wetting of solder.
Preferably, the above-mentioned solder extends from the pin pad beyond the circumferential edge of the shaft side plane of the enlarged diameter portion and spreads, through wetting, over the entire shaft side plane up to the shaft portion in such a manner as to form a fillet on the shaft side plane with an angle formed between the fillet surface and the shaft side plane being 5 degrees to 40 degrees.
According to the present invention, solder for joining the pin and the pin pad extends from the pin pad beyond the circumferential edge of the shaft side plane of the enlarged diameter portion and spreads, through wetting, over the entire shaft side plane up to the shaft portion. The fillet of solder formed on the shaft side plane is a relatively minor fillet in which an angle formed between the fillet surface and the shaft side plane is 5 degrees to 40 degrees.
Through employment of soldering as mentioned above, the joint portion between the pin and the pin pad exhibits far more enhanced joining strength; the pin becomes far more deformable; and the barycenter of the joint portion is lowered to a much greater extent. Thus, the substrate having pins of the present invention exhibits resistance to fracture potentially caused by stress imposed on a pin, to thereby enhance reliability.
Preferably, the pin is heat treated at a temperature of not lower than 600xc2x0 C., which is higher than a temperature at which the pin is joined to the pin pad by means of the aforementioned solder.
According to the substrate having pins of the present invention, the pin is heat treated at a temperature of not lower than 600xc2x0 C., which is higher than a temperature at which the pin is joined to the pin pad by means of solder; i.e., which is higher than the soldering temperature of the pin. A material wire from which the pin is formed, is formed through drawing to a predetermined diameter, during which the material wire is work-hardened due to residual strain associated with working. Also, when the enlarged diameter portion of the pin is formed and when the material wire is cut, strain associated with working is generated, and thus residual stress is generated, causing hardening of the pin. A temperature which is imposed on the thus-hardened pin during soldering (for example, approximately 200xc2x0 C. to 300xc2x0 C., 450xc2x0 C. to 500xc2x0 C. at the highest) is not sufficiently high to free the pin from work hardening. As a result, the pin fails to ease stress imposed on the pin through deformation. The stress tends to be imposed on the joint portion, potentially causing fracture of the joint portion.
According to the substrate having pins of the present invention, the pin is heat treated at a temperature of not lower than 600xc2x0 C., which is higher than a temperature at which the pin is joined to the pin pad by means of solder. Thus, no stress remains in the pin, to thereby impart flexibility to the pin. When stress is imposed on the pin, the pin can ease the stress through deformation, thereby preventing fracture of the joint portion and thus imparting enhanced joining strength to the substrate.
When the substrate is a resin substrate made of resin or a composite material which contains resin, use of a pin which is previously heat treated as mentioned above is particularly preferred, because a pin to which flexibility has been imparted through removal of residual strain associated with working can be used with even a resin substrate of low heat resistance which exhibits no resistance to a high temperature of not lower than 600xc2x0 C.; for example, a resin substrate resistant to a temperature of 300xc2x0 C. to 450xc2x0 C.
Thus it is an object of the invention is to provide an upright pin-joined substrate resistant to fracture potentially caused by stress imposed on a pin.
It is a further object of the present invention to provide a plate-like substrate having a plurality of upright pins secured by solder.
Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiments and appended drawings wherein like number refer to the same component element or feature.